Soils represent a large carbon pool, approximately 1500 Gt, which is equivalent to almost three times the quantity stored in terrestrial biomass and twice the amount stored in the atmosphere. Any modification of land use or land management can induce variations in soil carbon stocks, even in agricultural systems that are perceived to be in a steady state. Tillage practices often induce soil aerobic conditions that are favourable to microbial activity and may lead to a degradation of soil structure. As a result, mineralisation of soil organic matter increases in the long term. The adoption of no-tillage systems and the maintenance of a permanent vegetation cover using Direct seeding Mulch-based Cropping system or DMC, may increase carbon levels in the topsoil. In Brazil, no-tillage practices (mainly DMC), were introduced approximately 30 years ago in the south in the Paraná state, primarily as a means of reducing erosion. Subsequently, research has begun to study the management of the crop waste products and their effects on soil fertility, either in terms of phosphorus management, as a means of controlling soil acidity, or determining how manures can be applied in a more localised manner. The spread of no-till in Brazil has involved a large amount of extension work. The area under no-tillage is still increasing in the centre and north of the country and currently occupies ca. 20 million hectares, covering a diversity of environmental conditions, cropping systems and management practices. Most studies of Brazilian soils give rates of carbon storage in the top 40 cm of the soil of 0.4 to 1.7 t C ha-1 per year, with the highest rates in the Cerrado region. However, caution must be taken when analysing DMC systems in terms of carbon sequestration. Comparisons should include changes in trace gas fluxes and should not be limited to a consideration of carbon storage in the soil alone if the full implications for global warming are to be assessed. no-tillage / mulch / stocks / fluxes / greenhouse gas / Brazil

The Brazilian government aims at augmenting the area cropped under no-tillage (NT) from 32 to 40 million ha by 2020 as a means to mitigate CO 2 emissions. We estimated soil carbon (C) sequestration under continuous NT systems in two municipalities in the Goiás state that are representative of the Cerrado. A chronosequence of NT fields of different age since conversion from conventional tillage (CT) was sampled in 2003 and 2011. Soil C levels of native Cerrado and pasture were measured for comparison. After about 11 to 14 years, soil C stocks under NT were highest and at the levels of those under natural Cerrado. Average annual rates of soil C sequestration estimated using the chronosequence approach were respectively 1.61 and 1.48 Mg C ha −1 yr −1 for the 2003 and 2011 sampling, and were higher than those observed using repeated sampling after eight years. The diachronic sampling revealed that the younger NT fields tended to show higher increases in soil C stocks than the older fields. Converting an extra 8 million ha of cropland from CT to NT represents an estimated soil C storage of about 8 Tg C yr −1 during 10 to 15 years.At the Copenhagen Climate Change Conference of 2009 the Brazilian government committed to mitigation actions leading to projected reductions in its greenhouse gas emissions of 36 to 39% by 2020 1 . Brazil is one of the major greenhouse gas emitters in the world with historically more than half of its emissions originating from deforestation for agricultural land. From 2005 to 2010 reduced rates of deforestation resulted in circa 40% decrease in the total Brazilian emissions 2 . Today, emissions from agriculture, originating from cattle ranching and to a lesser extent from chemical fertilizer use, are at least equivalent to those from deforestation. Beside the continued efforts to control and reduce emissions from deforestation, one of the mitigation measures is the implementation of strategies that maintain and increase stocks of organic carbon (C) in agricultural soils. The adoption of no-tillage (NT) systems in croplands is considered as an effective way to achieve this. Within its ' Action Plan for Low Carbon Agriculture' launched in 2010, the Brazilian government aims at augmenting the area cropped under NT from 32 to 40 million ha by 2020 3 . Several studies suggest that a change from conventional tillage (CT) to NT cropping systems leads to an increase in soil C, especially if this occurs with intensification of crop production 4-7 .In Brazil, the Cerrado region is regarded as the target region for the efforts in mitigating CO 2 emissions through the adoption of NT systems 8 . The Cerrado occupies about 23% of the Brazilian territory or about two million km 2 in the central part of the country 9 . The region has experienced a rapid expansion of large-scale commercial agriculture since the early 1970s, when governmental policies were put in place designed to increase production of commodities for export as a response to the increased global demand for soybean and meat 10,11 . From a tot...

No-tillage cropping systems with direct seeding into a mulch of plant residues from cover crops -the so-called direct seeding mulch-based cropping (DMC) systems -have been adopted widely over the last 10-15 years in the Cerrado region of Brazil. They are replacing the traditional soybean monoculture with bare fallow using conventional tillage (CT) practices. The objective of this study was to examine how DMC practices affect soil organic carbon (SOC) dynamics and to assess their potential for enhanced soil carbon (C) storage. The approach was to determine soil C stocks along a chronosequence of fields under DMC, and then to apply the generic decomposition and yield (G'DAY) plant-soil model to analyse the soil C storage potential for a number of cropping systems. Forty-five fields were selected on a plateau of Ferralsols in the central Cerrado region to represent a chronosequence of 0-12 years under continuous DMC. Before DMC the fields had been under CT soybean monoculture following the clearing of the native savannah. An average increase in SOC stocks of 0.83 Mg C ha À1 yr À1 in the 0-20 cm topsoil was measured. The corresponding increase in total soil nitrogen was 79 kg N ha À1 yr À1 . The G'DAY model predicted a net accumulation of 0.70-1.15 Mg C ha À1 yr À1 in the 0-40 cm topsoil for the first 12 years, depending on the type of soil and DMC system. Model predictions showed that less soil C was accumulated under DMC systems that commenced immediately after clearing the native savannah. Gains in soil C under DMC were primarily due to the introduction of a second crop that caused higher net primary productivity, leading to higher plant C inputs to soil. A rough estimation shows that the conversion of 6 million ha of CT soybean monoculture to DMC in the Cerrados would enhance soil C storage by 4.9 Tg C yr À1 during at least the first 12 years following the conversion to DMC.

No abstract

The soil attributes controlling the CO2, and CH4 emissions were assessed in semiarid mangrove soils (NE-Brazil) under different anthropogenic activities. Soil samples were collected from different mangroves under different anthropogenic impacts, e.g., shrimp farming (Jaguaribe River); urban wastes (Cocó River) and a control site (Timonha River). The sites were characterized according to the sand content; physicochemical parameters (Eh and pH); total organic C; soil C stock (SCS) and equivalent SCS (SCSEQV); total P and N; dissolved organic C (DOC); and the degree of pyritization (DOP). The CO2 and CH4 fluxes from the soils were assessed using static closed chambers. Higher DOC and SCS and the lowest DOP promote greater CO2 emission. The CH4 flux was only observed at Jaguaribe which presented higher DOP, compared to that found in mangroves from humid tropical climates. Semiarid mangrove soils cannot be characterized as important greenhouse gas sources, compared to humid tropical mangroves.